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Abstract

In this work, we numerically investigate the radiative properties of metamaterial nanostructures made of two-dimensional tungsten gratings on a thin dielectric spacer and an opaque tungsten film from UV to mid-infrared region as potential selective solar absorbers. The metamaterial absorber with single-sized tungsten patches exhibits high absorptance in the visible and near-infrared region due to several mechanisms such as surface plasmon polaritons, magnetic polaritons, and intrinsic bandgap absorption of tungsten. Geometric effects on the resonance wavelengths and the absorptance spectra are studied, and the physical mechanisms are elucidated in detail. The absorptance could be further enhanced in a broader spectral range with double-sized metamaterial absorbers. The total solar absorptance of the optimized metamaterial absorbers at normal incidence could be more than 88%, while the total emittance is less than 3% at 100°C, resulting in total photon-to-heat conversion efficiency of 86% without any optical concentration. Moreover, the metamaterial solar absorbers exhibit quasi-diffuse behaviors as well as polarization independence. The results here will facilitate the design of novel highly efficient solar absorbers to enhance the performance of various solar energy conversion systems.

Figures (9)

(a) Schematic of proposed single-sized metamaterial solar absorbers made of 2D periodic
tungsten gratings with period Λ, patch width w and grating height
h, on a thin SiO2 spacer with thickness t and
an opaque tungsten thin film. The electromagnetic wave is incident at a polar angle
θ, polarization angle ψ, and azimuthal angle
ϕ. The structure is assumed to be geometrically symmetric in the x
and y directions, and ϕ is taken as 0° for simplicity. (b)
Schematic of double-sized metamaterial solar absorbers with tungsten patches of different
widths w1 and w2, and period
Λ′=2Λ Tungsten patches with the same size are arranged diagonally and
each patch is centered in its quadrant.

Contour plots of electromagnetic field distribution in the x-z cross-sectional view at the
middle of the tungsten patches, when (a) MP is excited at λ = 1.75
μm and (b) CMP is excited at λ = 0.78 μm for the
single-sized metamaterial absorber with Λ = 600 nm, w = 300 nm,
h = 120 nm, and t = 60 nm. Two unit cells are shown and
different layers are delineated. The contour represents the logarithmic values of magnitude
square of normalized magnetic field to the incidence, while the arrows indicate the electric
field vectors.

The resonance wavelengths of MP and CMP modes are summarized in (a−d) with varied
geometric parameters. The solid curves are predicted geometry-dependent resonance
wavelengths from the analytical LC model for the MP mode.

Comparison on the spectral normal absorptance between the double-sized metamaterial
absorber and multi-sized ones with three or four different tungsten patch widths. The insets
depict how to arrange the different patches such that they are diagonally symmetric. The
patch width values are: w1 = 250 nm,
w2 = 300 nm, w3 = 350 nm, and
w4 = 400 nm.